1. Field of the Invention
The present invention relates to a radial tire which has decreased rolling resistance and increased durability at high speeds, and which excels in handling stability and leads to a marked reduction in road noise.
2. Description of the Related Art
As vehicles have become more advanced and more high-quality, improvements in the mitigating of vibrations and in the comfort of the ride, especially in passenger vehicles, have progressed rapidly in recent years. Accordingly, low noise and a comfortable ride are required of tires as well.
In particular, a reduction in the noise which can be heard within the vehicle is desirable. An example of such noise is so-called road noise which is caused by the tires traveling over recesses and projections in the road surface while the vehicle is traveling, and the vibrations of the tires being transmitted such that the air within the vehicle vibrates. The demand for a reduction in road noise has become stronger in recent years.
Further, as vehicles have become more advanced, they have also become faster and have come to have a higher output. Conventional tires, in which only the comfort of the ride and a reduction in road noise have been improved, also must maintain high levels of durability at high speeds, uniformity, handling stability, and low rolling resistance.
The following are examples of basic, conventional methods for reducing road noise: (1) a method in which the rubber of the tire tread portion is softened; (2) a method in which the tension of the belt layers is increased by varying the configuration of the tire carcass; and (3) a method in which the rigidity of the belts in the circumferential direction thereof is increased by pressing an entire crossing belt layer or both end portions of the crossing belt layer with reinforcing layers having rubber-coated cords, e.g., nylon cords, disposed in the circumferential direction, or a method in which the reinforcing layers having cords are wound in spirals at the outer side of the belt layers so that there is no joint on the periphery (for example, Japanese Patent Application Laid-Open JP-A No. 6-24208).
Each of these methods has advantages and disadvantages.
The method to be used is selected in accordance with the purpose, and a combination of methods can be used. In particular, above method (3) is used as a method which improves durability at high speeds rather than a method which results in a reduction in road noise. This method (3) is currently and will most likely continue to be the main method used for high performance, high quality tires.
Special methods are known such as (4) a new method of sandwiching circumferential direction cords and a high modulus rubber sheet between the carcass layer and the belt layers as disclosed in JP-A No. 5-238205, and (5) a method of reinforcing portions corresponding to a tire vibration mode as disclosed in JP-A No. 3-253406. Further, the following methods are examples of application of a tire formed by above method (3), in which nylon cords are wound in spirals on the outermost layer of the belt in order to improve durability at high speeds and the high speed level in particular: (6) a method of winding cords having a high modulus of elasticity (e.g., JP-A No. 2-147407, JP-A No. 1-145203), and (7) a method which, in order to improve the vulcanization molding, uses composite cords in which the cords wound in spirals around the outermost layer are twisted together with cords having a high modulus of elasticity and cords having a low modulus of elasticity, the stress-strain curve of the composite cords having a point of inflection (e.g., JP-A No. 1-247204). Many other methods are known such as (8) a method to improve the noise of the tire formed by above method (7) by limiting the fiber materials which are twisted together (JP-A No. 6-305304), and (9) a method in which organic fibers are used in reinforcing layers at both side portions of the belt layers in the radial direction of the tire (JP-A No. 6-115312), and the like. In all of the methods described above, whether explicitly disclosed in these publications or not, the tension of the belt portion is slightly strengthened, and therefore, there is a slight reduction in road noise.
However, in method (1), even if road noise can be reduced by softening the tread rubber, this method is not practical because wear resistance markedly decreases and the handling stability deteriorates greatly. In method (2), even though the tension of the belt layers of the tire can be increased, the transverse rigidity and the cornering performance of the tire deteriorates, and portions other than the tread portion contact the ground such that the external appearance of the tire is not satisfactory. Further, in method (3), although there is a slight improvement in durability at high speeds and a slight reduction in road noise, this improvement and this reduction are only slight and therefore are not satisfactory. Methods (4) and (5), which are applied examples of method (3), have the same aforementioned effects as or better effects than method (3), but the rate of improvement brought about by methods (4) and (5) is slight, and further, there is not sufficient durability at high speeds. It is difficult to manufacture tires by using method (6), and further, method (6) results in a deterioration in handling stability. Because there is a point of inflection in the stress-strain curve in methods using composite cords such as methods (7) and (8), the reduction in road noise in the large range of 100 to 500 Hz is insufficient because the performance of the cords differs at large inputs and small inputs. Moreover, these methods are not preferable from a practical standpoint because the effect of the composite cords on the reduction of road noise is highly dependent on the speed of the vehicle. JP-A No. 6-115312 which discloses method (9) does not specify the cord characteristics, and although there is a slight reduction in road noise with method (9), the reduction is insufficient. Further, because the cord characteristics are not completely generated by the tire characteristics, the reduction in road noise is insufficient. When cords having a high modulus of elasticity and formed from an ordinary aromatic polyamide (such as the cords of the only Example of JP-A No. 6-115312 which discloses this method (9)) are wound in spirals, the road noise cannot be reduced sufficiently, and the handling stability greatly deteriorates.
As described above, there are no conventional tires having sufficient durability at high speeds and handling stability (which can be considered to be essential factors of the required performances of current tires), and also able to greatly reduce road noise.